Infrared observations of Phobos and Deimos from Viking

The surface thermal properties of Phobos and Deimos have been determined from observations made with the Viking Orbiter Infrared Thermal Mapper (IRTM), at wavelengths ranging from 6 to 20 μm. The data, composed of both global and high-resolution infrared photometry of the satellite surfaces as well as eclipse observations, indicate surface material of low thermal conductivity comparable to that of the earth's moon. Values of the thermal inertia I consistent with the data for Phobos are 0.9 ≲ I ≲ 1.6 × 10^(−3) cal cm^(−2) s^(−1/2) K^(−1), and 0.6 ≲ I ≲ 2.0 × 10^(−3) cal cm^(−2) s^(−1/2) K^(−1) for Deimos. It is concluded that both satellites are covered with a vertically uniform layer of finely divided material at least several centimeters thick. Observed differences between brightness temperatures at different wavelengths on Phobos are due mainly to topographic slopes and to the presence of ∼5% by area high inertia or blocky material.